An injection device of this kind is known from German Patent 36 38 984. This patent relates to a fully automatic injector with a spring system. In the known device, it is possible to vary the operative length of the plunger in all operating positions. Hence if someone plays with the device which can never be precluded--the risk arises that by turning it, he or she will change the length of the plunger, which can lead to incorrect dosages.
Another injection device of the type discussed at the outset is known from WO 88.08725. Once again, this is a fully automatic injector with a spring system, and in this known device as well it is possible to vary the effective length of the plunger in all operating positions, that is, including in the inactive position. In this device this is necessary to allow some injection liquid to be expelled in the inactive position, for monitoring purposes. However, if it is played with, for instance by children, it might be possible under some circumstances for a large amount of insulin to be expelled by mistake. This known device has the advantage that--as a result of various safety provisions--actual dose setting is not possible until the device is in its cocked position. This is achieved at the cost of relatively complicated operation.